Atmospheric Aerosols: Chemistry, Clouds, and Climate

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Abstract

Atmospheric aerosol particles impact Earth’s radiation balance, and therefore its climate, both
directly by scattering and absorbing solar radiation and indirectly by influencing cloud albedo.
By their direct and indirect effects on climate, aerosols provide a net negative radiative forcing
that may be comparable in magnitude to the positive forcing by CO2. However, significant gaps
in our scientific understanding of aerosol-related climate forcings have resulted in very large
uncertainties in the estimates of their magnitudes. Organic matter is a ubiquitous component of atmospheric aerosols, typically comprising 10 to
90% of fine aerosol mass. Inorganic aerosols may acquire an organic component via in situ
interactions with volatile organic compounds (VOCs), a family of processes known as secondary
organic aerosol (SOA) formation. SOA formation is one of the greatest sources of uncertainty in
estimations of aerosol forcing on climate. Our recent work provides evidence that SOA
formation may significantly change the climate properties of the seed aerosol [Shapiro et al.,
2009; Sareen et al., 2010]. We have identified via laboratory studies that particle-phase chemical
reactions of the -dicarbonyl species glyoxal and methylglyoxal with ammonium salts may
result in secondary products which absorb light in the UV and visible. We have also observed
that methylglyoxal depresses surface tension in aqueous aerosol mimics by up to 45%. These
observations have potentially significant implications for our understanding of the effects of
secondary organic aerosol material (SOA) on climate, since a) SOA is typically treated as nonabsorbing
in climate models, and b) surface active SOA material may alter the ability of an
aerosol particle to nucleate and grow into a cloud droplet.